Field of the Invention
In the production of acrylonitrile by the catalytic ammoxidation of propylene with ammonia and oxygen, a crude acetonitrile co-product is produced. The term "crude acetonitrile" means liquid acetonitrile containing hydrogen cyanide, water and other impurities. The other impurities may include acrylonitrile, acetaldehyde, acetone, methanol, acrolein, oxazole, cisand trans-crotononitrile, methacrylonitrile and allyl alcohol. The relative proportions of the components of the crude acetonitrile can vary over a wide range depending on various conditions. The concentration level of the organic impurities in the crude acetonitrile is usually less than 15% with no single organic component found in greater than 2 to 4 wt % concentration. Usually crude acetonitrile obtained from an acrylonitrile plant contains between 25 and 85% acetonitrile. Typically, the crude acetonitrile is composed on a weight basis of 52% acetonitrile, 43.6% water, 2.5% hydrogen cyanide, 0.5% acrylonitrile and 1.3% other organic impurities as mentioned above. Originally, during the manufacture of acrylonitrile, the crude acetonitrile produced was disposed of by incineration. However, in recent years, this material has been recovered and purified and sold to add value to the process.
There are two basic technologies for the "first stage" purification of crude acetonitrile co-product produced during the manufacture of acrylonitrile. These processes typically produce acetonitrile of sufficient purity for use as a bulk solvent. The first and most common practiced technology is a batch process. In this process, crude acetonitrile is distilled to remove the bulk of the HCN as a low boiling distillate. The remaining material is allowed to react either with a mixture of strong base, usually an aqueous sodium hydroxide solution and formaldehyde and water, or with a strong base and ferrous sulfate, to remove essentially all the remaining HCN. (See U.S. Pat. Nos. 4,328,075 and 3,201,451.) The HCN free material is then distilled to produce an acetonitrile/water azeotrope containing about 25% water. The azeotrope is then slurried with anhydrous calcium chloride to remove the bulk of the water in the azeotrope and produce an acetonitrile/water mixture containing about 3 to 5% water. This mixture is then distilled to produce acetonitrile product having an acceptable purity for many uses. Typically, this material contains several parts per million by weight of acrylonitrile or other impurities which absorb strongly in the UV spectrum.
The second method of producing "first stage" purified acetonitrile is a continuous recovery process which involves (1) first distilling crude acetonitrile in a distillation zone at a pressure of at or above 1 atmosphere to remove the bulk of the HCN, (2) passing this azeotrope through a digester in which the remaining HCN is removed by treatment with an aqueous solution of base and formaldehyde, (3) performing a second distillation at a pressure less than 1 atmosphere to separate the material into a bottoms product containing water and a second acetonitrile/water azeotrope with higher acetonitrile concentration, and (4) a third distillation at a pressure above the pressure of the first distillation to produce purified acetonitrile as a side stream. This process is described in U.S. Pat. No. 4,362,603 assigned to the assignee of the present invention and herein incorporated by reference. Acetonitrile purified by this method can contain up to several parts per million by weight of acrylonitrile, acetamide, oxazole, or other UV-absorbing impurities.
While these two basic procedures for forming bulk solvent grade acetonitrile are widely accepted, their use in producing acetonitrile for chromatographic applications is not acceptable because the acetonitrile contains a relatively high amount of UV-absorbing impurities. There is a distinct market for high performance (HPLC grade) acetonitrile essentially free of UV-absorbing impurities (UV cutoff for impurities of &lt;190nm).
The specifications for HPLC grade acetonitrile render the material produced by first stage purification unacceptable, therefore requiring further processing by producers of HPLC grade acetonitrile. The traditional commercial methods of acetonitrile purification to achieve this high grade of purity utilize costly multi-step processes involving, for example, permanganate oxidation, acid treatment, phosphorous pentoxide drying and two distillations.
More recent procedures disclosed in U.S. Pat. Nos. 5,292,919 and 5,426,208 disclose treatment of acetonitrile with ozone followed by passing the acetonitrile through a series of adsorbent beds of charcoal or graphitized carbon, activated alumina, and/or molecular sieves.
Several other patents and literature articles describe the purification of acetonitrile with acidic ion exchange resins for removal of impurities from acetonitrie. British Patent 1,223,915 describes the use of a series of strong acid cation exchange resins for the reduction of the concentration of bases, ammonia and 3,3'-iminodipropionitrile, in aqueous acetonitrile from 500 ppm each to 10 ppm and &lt;50 ppm, respectively. This level of purity is still not acceptable for HPLC grade acetonitrile.
The process of the present invention is directed to an improved procedure for easily removing substantially all of the acrylonitrile impurity present in crude acetonitrile, in particular, crude acetonitrile produced as a co-product during the manufacture of acrylonitrile. In addition, the process of the present invention results in the production of a crude acetonitrile intermediate stream which simplifies the process for producing HPLC grade acetonitrile resulting in substantial economic savings.